glCopyConvolutionFilter2D defines a two-dimensional convolution
filter kernel with pixels from the current GL_READ_BUFFER (rather than
from main memory, as is the case for glConvolutionFilter2D).

The screen-aligned
pixel rectangle with lower-left corner at (x, y), width width and height
height is used to define the convolution filter. If any pixels within this
region are outside the window that is associated with the GL context,
the values obtained for those pixels are undefined.

The pixels in the rectangle
are processed exactly as if glReadPixels had been called with format set
to RGBA, but the process stops just before final conversion. The R, G,
B, and A components of each pixel are next scaled by the four 2D GL_CONVOLUTION_FILTER_SCALE
parameters and biased by the four 2D GL_CONVOLUTION_FILTER_BIAS parameters.
(The scale and bias parameters are set by glConvolutionParameter using
the GL_CONVOLUTION_2D target and the names GL_CONVOLUTION_FILTER_SCALE
and GL_CONVOLUTION_FILTER_BIAS. The parameters themselves are vectors of
four values that are applied to red, green, blue, and alpha, in that order.)
The R, G, B, and A values are not clamped to [0,1] at any time during this
process.

Each pixel is then converted to the internal specified by internalformat.
This conversion simply maps the component values of the pixel (R, G, B,
and A) to the values included in the internal (red, green, blue, alpha,
luminance, and intensity). The mapping is as follows:

Internal Format Red Green Blue Alpha Luminance Intensity

GL_ALPHA A

GL_LUMINANCE R

GL_LUMINANCE_ALPHA A R

GL_INTENSITY R

GL_RGB R G B

GL_RGBA R G B A

The red, green, blue, alpha, luminance, and/or intensity components of
the resulting pixels are stored in floating-point rather than integer

Note that after a convolution is performed,
the resulting color components are also scaled by their corresponding GL_POST_CONVOLUTION_c_SCALE
parameters and biased by their corresponding GL_POST_CONVOLUTION_c_BIAS
parameters (where c takes on the values RED, GREEN, BLUE, and ALPHA). These
parameters are set by glPixelTransfer.

GL_INVALID_ENUM is generated if internalformat
is not one of the allowable values.

GL_INVALID_VALUE is generated if width
is less than zero or greater than the maximum supported value. This value
may be queried with glGetConvolutionParameter using target GL_CONVOLUTION_2D
and name GL_MAX_CONVOLUTION_WIDTH.

GL_INVALID_VALUE is generated if height
is less than zero or greater than the maximum supported value. This value
may be queried with glGetConvolutionParameter using target GL_CONVOLUTION_2D
and name GL_MAX_CONVOLUTION_HEIGHT.

GL_INVALID_OPERATION is generated if
glCopyConvolutionFilter2D is executed between the execution of glBegin
and the corresponding execution of glEnd.